#137 Add plexon 1 test file

This is the file provided by Plexon. The problem is that the current file in the repository has only one stream with very idiosyncratic channel names. The file offered by plexon has more than one stream with standard channel names which is important to test for.

I also added better descriptions of the files in the plexon folder.

Looks good to me! Maybe @zm711 can also take a look?

Looks good to me too. I'll merge :)